The present invention relates to methods and apparatus for starting a plasma arc treatment system. Plasma arc treatment systems advantageously attain high temperatures (10,000.degree. C.-15,000.degree. C.) with a non-contaminating electrical heat source and are used in applications such as metal melting, powder production, and hazardous waste incineration.
In plasma arc treatment chambers, a plasma torch transfers electrical energy through a stream of ionized gas so that the gas becomes an electrical conductor. The commonly owned U.S. Pat. No. 4,912,296, for example, discloses an advantageous construction for a plasma torch incinerator. U.S. Pat. No. 4,770,109 and U.S. Pat. No. 5,136,137, both by the inventor of this application and also commonly owned, disclose and claim reactors for the incineration and melting of all types of materials, particularly hazardous waste, for which the invention of this application is particularly useful. The disclosure of the commonly owned patents is incorporated herein by reference. Other patents relating to the field of materials incineration and melting include U.S. Pat. Nos.:
______________________________________ 3,599,581 4,432,942 3,779,182 4,582,004 4,181,504 4,615,285 4,326,842 4,918,282 and British Patent No. 1,170,548. ______________________________________
When used as a hazardous waste incinerator, the plasma torch raises the temperature of the waste materials to such high levels that the waste materials chemically break down (pyrolysis). The breakdown is enhanced by maintaining an atmosphere of appropriate gas in the incinerator. As a result, the residues are usually harmless gases and solids which can be suitably removed from the incinerator. The term hazardous waste as used herein refers to any nuclear, toxic, chemical and/or biological waste.
Generally speaking, there are two types of plasma torches: non-transferred and transferred arc torches. In non-transferred arc torches, the electrical potential is contained entirely within the plasma torch; for example, between two coaxial rings so that an electrical arc forms in the annular space between the coaxial rings. A gas is passed through the annular area and emitted from an end of the torch.
In transferred arc torches, the torch acts as one side of the electrical field and the other side of the field is exterior of the torch and spaced apart from the torch. Transferred arc torches are preferable since they are more efficient and attain higher operating temperatures than non-transferred arc torches. U.S. Pat. No. 5,136,137, by the inventor of this application and commonly owned, discloses a transferred arc torch. Referring to FIG. 1 of U.S. Pat. No. 5,136,137, the reactor includes a hollow containment vessel. A conical, electrical conducting member is attached to the bottom of a drum and a plasma torch is mounted to the top of the vessel. A high voltage terminal is coupled to the torch so that an electrical potential exists between the conducting member and torch. During operation, an arc is sustained between the torch and electrical conducting member forming a plasma plume.
Plasma arc treatment systems typically operate with a liquid slag in the drum. The molten slag insulates the refractory from the hot plasma and can also be used to increase the conductivity between the torch and the grounding network. When the materials being treated are inorganic, the molten slag is usually an effective conductor. If only organic matter is being treated, slag formers, such as sand and soda ash, may be added to increase the conductivity of the slag.
When the plasma arc treatment system is shut down, an amount of the slag is left in the drum to form the slag for a subsequent use. During the down time the slag cools and may solidify. As the slag temperature drops, the conductivity of the slag also decreases. A problem that occurs when starting the plasma arc treatment system is that the conductivity of the slag may have dropped to a level which will not sustain an arc between the torch and grounding network. In order to start the treatment system and sustain an arc, the slag must be heated to increase the conductivity.
A conventional method of heating the slag is with an oxyacetylene torch or a non-transferred plasma arc torch. A problem with the conventional method of heating the slag is that a considerable amount of time is required to heat the slag. The torch is generally applied to the surface of the slag and the heat tends to dissipate throughout the slag and containment vessel.